In the course of manufacturing grain oriented silicon-iron electrical steels, a forsterite coating is formed during the high temperature annealing process. Such forsterite coatings are well-known and widely used in prior art methods for the production of grain oriented electrical steel. Such coatings are variously referred to in the art as a “glass film”, “mill glass”, “mill anneal” coating or other like terms and defined by ASTM specification A 976 as a Type C-2 insulation coating.
A forsterite coating is formed from the chemical reaction of the oxide layer formed on the electrical steel strip and an annealing separator coating, which is applied to the strip before a high temperature anneal Annealing separator coatings are also well-known in the art, and typically comprise a water based magnesium oxide slurry containing other materials to enhance its function.
After the annealing separator coating has dried, the strip is typically wound into a coil and annealed in a batch-type box anneal process where it undergoes the high temperature annealing process. During this high temperature annealing process, in addition to the forsterite coating forming, a cube-on-edge grain orientation in the steel strip is developed and the steel is purified. There are a wide a variety of procedures for this process step which are well established in the art. After the high temperature annealing process is completed, the steel is cooled and the strip surface is cleaned by well-known methods that remove any unreacted or excess annealing separator coating.
In most cases, an additional coating is then applied onto the forsterite coating. Such additional coatings are described in ASTM specification A 976 as a Type C-5 coating, and often described as a “C-5 over C-2” coating. Among other things, a C-5 coating (a) provides additional electrical insulation needed for very high voltage electrical equipment which prevents circulating currents and, thereby, higher core losses, between individual steel sheets within the magnetic core; (b) places the steel strip in a state of mechanical tension which lowers the core loss of the steel sheet and improves the magnetostriction characteristic of the steel sheet which reduces vibration and noise in finished electrical equipment. Type C-5 insulation coatings are variously referred to in the art as “high stress,” “tension effect,” or “secondary” coatings. Because they are typically transparent or translucent, these well-known C-5 over C-2 coatings, as used on grain oriented electrical steel sheets, require a high degree of cosmetic uniformity and a high degree of physical adhesion in the C-2 coating. The combination of the C-5 and C-2 coatings provide a high degree of tension to the finished steel strip product, improving the magnetic properties of the steel strip. As a result, improvements in both the forsterite coating and applied secondary coating have been of great interest in the art.